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According to the June 6, 2001 issue of the New England Journal of Medicine, women with hypothyroidism who are taking thyroid hormone replacement medication may need an increased dosage if the begin estrogen treatment after menopause.

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In a study conducted by endocrinologist Baha Arafah, of University Hospitals of Cleveland, he found that the additional thyroid medicine needed was "small but potentially clinically important" when women began taking the female hormone estrogen for hot flashes, excessive sweating and to treat or prevent brittle-bone disease.

Dr. Arafah told Reuters that an estimated 5 to 10 percent of the women aged 50 to 55 have thyroid problems and that as many as half of them may be receiving estrogen for menopausal symptoms. That translates to from 400,000 to 800,000 U.S. women in that age group alone who may be taking both estrogen and thyroid hormone replacement. Dr. Arafah estimates that worsening hypothyroidism may become clinically important in as many as 40 percent of thyroid patients beginning estrogen treatment for menopause.

In hypothyroid women studied, estrogen treatment led to decreases in serum free thyroxine (free T4) concentrations that were in some cases enough to warrant an increased thyroid hormone replacement dosage. Women with normal thyroid function did not have similar thyroid function changes.
In a related editorial, Robert D. Utiger, M.D. states his conclusion, that:

Because women with hypothyroidism who are taking thyroxine may need more thyroxine when they are treated with estrogen and may need less thyroxine after estrogen is discontinued, it is prudent to reassess their thyroid function several months after estrogen therapy is either initiated or discontinued.

Dr. Arafah advises women who are hypothyroid to have their thyroid function tested 12 weeks after they begin estrogen therapy, to assess if additional thyroid medicine is needed.

Note to patients: Since the estrogen concentration found in hormone replacement is less than is found in birth control pills, it's safe to assume that hypothyroid patients who are beginning birth control pills should also have thyroid function retested after 12 weeks.

And, since pregnant women with hypothyroidism who are treated with thyroxine require an average increase of 45 percent in the dose of thyroxine to maintain a normal thyroid level, the estrogen changes that occur during pregnancy should always warrant frequent and close monitoring of thyroid levels, in order to adjust dosages for optimal health of both mother and baby.

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